1. Technical Field
The present disclosure relates to compensating for combusting oxygenated fuels in a diesel engine.
2. Background Art
Diesel engines are carefully calibrated to achieve mandated emission standards, but without substantially harming fuel economy and performance. To that end, exhaust gas recirculation (EGR), the number and timing of fuel injection pulses, turbocharger boost pressure, and other engine parameters are controlled to achieve the simultaneous goals. Such combination of control parameters is determined for standard diesel fuel. In the interest of supplanting fossil fuel usage, biodiesels and other alternative fuels are being developed and proposed as blending agents in diesel fuel. Many alternative fuels contain oxygen, i.e., oxygenated fuels. Some biodiesels are oxygenates, which are made by reacting oils, such as vegetable oil or animal fat, with an alcohol. One class of such biodiesels are fatty acid methyl esters (FAMEs), with canola-based, industrial-rapeseed-based, and soy-based FAMEs as examples. Other examples of oxygenates include alcohols and ethers. Oxygenates have a lower energy content than diesel fuel, the latter containing almost entirely hydrogen and carbon with only trace amounts of oxygen and other elements. When an oxygenate or oxygenate blend is directly substituted for diesel fuel at the same operating condition otherwise, the engine produces less power. The vehicle operator may compensate for the loss of power by requesting a higher level of power by depressing the accelerator pedal farther. However, not only does this cause the vehicle operator to sense a loss of power in the vehicle, but, the emissions and fuel economy suffer by relying on depressing the pedal farther to achieve the desired torque. In the absence of information that the fuel is an oxygenate, the engine's control system assumes that the pedal being actuated to a greater extent signifies that the torque level being requested is higher than what the operator actually desires. The EGR, turbocharger boost, number and timing of injections, and other engine parameters are being commanded based on the assumption that the driver is requesting a higher torque level and the fuel is diesel. When the fuel being supplied has a lower energy content than expected, the commanded engine parameters, based on the greater pedal actuation, are inappropriate for oxygenated fuel, which leads to increased emissions, reduced fuel economy, noisy combustion, and/or other undesirable consequences.
Furthermore, a diesel engine is typically calibrated to limit the total amount of fuel that can be injected into the engine to stay within physical design limits of the engine, i.e., to maintain peak pressure, turbine inlet temperature and/or turbine speed within component design limits. When an oxygenate or diesel/oxygenate blend is subjected to such limitation and the fuel injection pulses are not adjusted to account for the oxygen content in the fuel blend, the peak power from the engine is reduced, leading to customer dissatisfaction.
Another issue associated with using oxygenates arises due to their lesser range in volatility. Some exhaust aftertreatment devices operate more efficiently in a particular temperature range. Some exhaust aftertreatment devices require some unburned, or partially combusted, fuel for purposes of converting collected pollutants to unregulated components during a periodic regeneration process. According to some regeneration strategies, a post-injection fuel pulse is provided to cause late combustion to increase exhaust temperature and/or to provide hydrocarbons into the exhaust stream. As the post-injection pulse occurs late in the cycle when the piston is at a lower position, a significant fraction of the fuel sprays on cylinder walls and mixes with the oil on the cylinder walls. The piston rings then scrape the fuel and oil into the crankcase oil. Dilution of the oil by fuel negatively impacts the oil's ability to provide lubrication to the engine. In some cases, this leads to high oil levels causing oil to be pulled out of the crankcase and inducted into the engine, which can foul the intake and lead to unintended acceleration when combusted in the engine. Some engine controllers estimate oil dilution and at least partially base recommended oil change interval on the oil dilution level. The factors that can be used to estimate the amount of fuel in the oil include: the number, timing, and quantity of fuel injected in a post-injection pulse and the subsequent engine operating conditions. That is, in between events, such as regenerations of an aftertreatment device that calls for post-injections, some of the fuel in the oil vaporizes at engine operating conditions leading to elevated oil temperatures. Diesel fuel has a relatively wider range of volatility characteristics than oxygenates such that the lighter ends of the diesel fuel does vaporize. In contrast, oxygenates have a narrower range of volatility such that very little of the oxygenate is highly volatile so that very little of the oxygenate, once diluted with oil, is removed during subsequent engine operation.
Another potential issue with biodiesels arises when trying to start an engine at low temperatures when there is a nontrivial amount of biodiesel blended in diesel fuel. Biodiesel has a greater tendency to wax. If starting is attempted with fuel, which is too waxy, the fuel injection system can be damaged.